I’m not a true expert here but my observation is that
(x) you have ample capital, and
(y) want to do something reasonably hard, where you need a whole bunch of very talented, experienced web engineers,
(z) which are impossible to get, especially fast
(aa) Pivotal Labs is one of the very top places to go.
They’ve created a great culture where lots of high quality engineers go instead of doing a “start-up”. The key is lots. There are a number of boutique shops, but if you need what are really outsourced resources … most boutiques can’t get you enough throughput.
Pivotal Labs can, near as I’ve observed.
You gotta pay, though.